This invention relates to ac electrical protective thermal relay devices responsive to overload current or heat generation in a motor or other electrical unit requiring protection therefrom and which is also responsive to actuation by a detection device that senses some other abnormal condition that must be protected against.
In general, thermal relays comprise a heater through which loaded current flows and a bimetallic switch to be heated by said heater. Conventionally, the thermal relay is housed in the device to be protected as, for example, a motor so it may be effective in protecting the device against overcurrent when overloaded.
The desire to stop the motor may arise not only from the motor's overcurrent flow or its generated-heat but from water level reduction for an underwater pump or shortage of refrigerant in air conditioning equipment. For those cases, shutting off the motor's power by means of the thermal relay instead of other kinds of switch element would result in substantial advantage.